This section introduces the following terminology used in voice traffic engineering:

Blocking probability

Grade of Service (GoS)


Centum Call Second (CCS)

Busy hour

Busy Hour Traffic (BHT)

Call Detail Record (CDR)

Blocking Probability and GoS


The blocking probability value describes the calls that cannot be completed because insufficient lines have been provided. For example, a blocking probability value of 0.01 means that 1 percent of calls would be blocked.

GoS is the probability that a voice gateway will block a call while attempting to allocate circuits during the busiest hour. GoS is written as a blocking factor Pxx, where xx is the percentage of calls that are blocked for a traffic system. For example, traffic facilities that require P01 GoS define a 1 percent probability of callers being blocked.

The number of simultaneous conversations affects the voice traffic. Users vary widely in the number of calls they attempt per hour and the length of time they hold a circuit. Any user's attempts and holding times are independent of the other users' activities. A common method used to determine traffic capacity is to use a call logger to determine the number of simultaneous calls on the network and then determine the probability that exactly x simultaneous calls will occur. Voice systems can be provisioned to allow the maximum number of simultaneous conversations that are expected at the busiest time of the day.


The Erlang is one of the most common measurements of voice traffic.

KEY One Erlang equals one full hour, or 3600 seconds, of telephone conversation. POINT

For example, if a trunk carries 12.35 Erlangs during an hour, an average of a little more than 12 lines (connections) are busy. One Erlang indicates that a single resource is in continuous use. The traffic measurement in Erlangs is used to determine whether a system has too many or too few resources provisioned.


A CCS represents 1/36th of an Erlang.

NOTE Centum means one hundred.

A system port that can handle a continuous one-hour call has a traffic rating of 36 CCSs (or 1 Erlang). Station traffic varies greatly among users, but the typical range is approximately 6 to 12 CCSs per port. If no exact statistical data exists, assume that the average typical trunk traffic is 30 CCSs per port.

For example, one hour of conversation (one Erlang or 36 CCSs) might be ten 6-minute calls or 15 4-minute calls. Receiving 100 calls, with an average length of 6 minutes, in one hour is equivalent to ten Erlangs, or 360 CCSs.

Busy Hour and BHT


The busy hour is the 60-minute period in a given 24-hour period during which the maximum total traffic load occurs. The busy hour is sometimes called the peak hour.

The BHT, in Erlangs or CCSs, is the number of hours of traffic transported across a trunk group during the busy hour (the busiest hour of operation).


To calculate the BHT in Erlangs, multiply the number of calls in the busiest hour by their average duration in seconds, and divide the result by 3600.

To calculate the BHT in CCSs, multiply the number of calls in the busiest hour by their average duration in seconds, and divide the result by 100.

For example, if you know from your call logger that 350 calls are made on a trunk group in the busiest hour and that the average call duration is 180 seconds, you can calculate the BHT as follows:

■ BHT = Average call duration (seconds) * calls per hour/3600

A CDR is a record containing information about recent system usage, such as the identities of sources (points of origin), the identities of destinations (endpoints), the duration of each call, the amount billed for each call, the total usage time in the billing period, the total free time remaining in the billing period, and the running total charged during the billing period. The format of a CDR varies among telecom providers and call-logging software; some call-logging software allows the user to configure the CDR format.

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